Our broad goal is to advance understanding of the strategies used by the central nervous system to organize muscular activity in accomplishing motor tasks. It is evident that there must be some provision for the functional organization of motor units, and our proposal is designed to test one possible scheme. We propose that the nervous system organizes motor units into cohesive groups that we call motor-unit synergies. In our model, each synergy is assembled from motor units that belong to more than one muscle but need not include all motor units in any one muscle. Motor unit members of each synergy are recruited in order by a modified version of the size principle. A significant advantage of this scheme is that motor units in the synergy are self-organizing, i.e.they initiate and co-modulate their firing in an orderly fashion as a consequence of their intrinsic properties and the common synaptic input that assembles them. We propose to test for motor-unit synergies by obtaining simultaneous records from approximately 20 motor units from selected muscles in sentient cats during treadmill locomotion. Specific Aim 1 is to test the hypothesis that the motor-unit synergy we find in reflexes is expressed in voluntary movements. Specific Aim 2 is to test the hypothesis that synergies are formed by the motor units of muscles that-are co-active but distinctive in their independent mechanical actions. Specific Aim 3 is to test hypothesis that proprioceptive feedback contributes to the formation of motor-unit synergies. Meeting these aims will identify the rules for organization for motor units that are active throughout a limb. Motor units may be organized into synergies of their own or as part of a muscle-based system. Our findings will distinguish these two strategies or reveal conditions under which they switch or superimpose. In any case, our studies will provide a more global view of the operation of motor units than is currently available. When this information is obtained it will be valuable to determine whether the functional organization of motor units can be modified to acquire new skills or adapt to injury.